This paper presents a system architecture to address the traffic density and the network coverage open research challenges in vehicular networks. This architecture adopts mobile edge computing (MEC) and software-defined networking (SDN) technologies to increase the overall network reliability and scalability under high traffic density conditions. Then, a device-to-device (D2D) clustering method is described to provide network coverage for orphan nodes. The proposed architecture offers a reliable structure to support ultra-low latency applications. Evaluation results using realistic conditions for various network scenarios show that the proposed MEC/SDN-enabled vehicular network architecture achieves performance gains of up to 74% in terms of task blocking compared to its baseline implementation.